Add flock into the link line on make builds to seralize links.
Add flock into the link line on make builds to seralize links.

This is because linking is often memory bound, so running only one
at a time is a good idea.
Review URL: http://codereview.chromium.org/2891027

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp